EP 1 407 935 A1 and the related patent family describes a process for applying a thin ceramic coating material to a surface of a motor vehicle assembling element made of aluminum, which is to be coated, wherein said aluminum is anodized before being coated, and a roughness of the surface to be coated for adhesion of the coating material is achieved by said anodizing process. Then, said thin ceramic coating material, which exclusively consists of inorganic components, is applied by means of an electrostatic application method or by means of a wet-chemical application method at an almost constant layer thickness as a coating with a pore-free and closed surface.
This technical teaching is based on the object of improving the quality of known thin ceramic coatings. In particular, a process is to be provided that enables a cost efficient production of high quality thin ceramic coatings. In addition, parts or objects that have a high quality thin ceramic coating and can be produced cost efficiently are to be created. It is further essential that the thin ceramic coating exclusively consists of inorganic components. The description of the process ends with the application of the coating to the aluminum surface.
WO 2009/068168 A2 and the related patent family describe a component made of aluminum and/or an aluminum alloy, particularly a decorative or functional part, having very high corrosion resistance, and to a method for the production thereof. The conversion layer is to be sealed in the course of at least 3 min/μm of layer thickness. The high corrosion resistance, particularly high alkali resistance, is to be achieved in that the surface of the component comprises an oxide layer created evenly by anodization and a cover layer sealing and evenly covering the porous oxide layer. The cover layer is created by an oxide layer hydrate compound sealing the pores of the oxide layer and by an additional inclusion of glass-like substances and application thereof to the oxide layer at the same time. A compound of one or more alkali silicates is proposed as said glass-like substances. Alternatively, the cover layer may also comprise exclusively aluminum oxide and/or aluminum hydrates and/or aluminum oxide hydrates and/or alkali silicates and/or alumosilicates.
WO 2011/020556 A1 and the related patent family also describe an aluminum or aluminum alloy formed and/or structural part, and a process for protecting its surface. An anti-corrosion layer obtained from a sol-gel system is applied directly to the surface of aluminum or aluminum alloy, without anodized layer, which is to be produced by integrated hardening or drying during an optimized process sequence, that is, a shortened process sequence.
An anodized layer is also omitted in EP 2 328 183 A1 and the related patent family. In a substrate with a metal foil for preparing photovoltaic cells, a first side of the metal foil is provided for arranging a photovoltaic-absorber layer. In order to improve the chemical resistance and the corrosion resistance at elevated temperature, a protective layer of a silicon-based sol-gel paint is arranged on the second side of the metal foil.
EP 1 306 467 A1 describes a thermoplastic resin-coated aluminum plate, wherein the aluminum plate bears a semi-non-porous conversion layer prepared by a pretreatment. In [0012], the term “semi-non-porous” is characterized in that the ratio (called porosity) of the free areas of pores present in the conversion layer on the surface of the aluminum plate to the total area of the anodized film is 30% or less. If the porosity is 5% or less, the film is called practically non-porous. The thickness of this layer can be within a range of from 50 to 3000 Å (5 to 300 nm). According to [0031], the conversion layer is coated with a polymer containing silicon. This polymer has corresponding thermoplastic properties and is prepared from various silanes or siloxanes as precursors.
JP 06-316787 A describes the anodization of an aluminum surface by immersing it into a water-containing alcoholic HCl solution containing a small amount (<2% by weight) of an alkoxysilane to obtain a fully sealed conversion layer.
JP 60-179475 A describes the formation of a conversion layer on aluminum surfaces by applying an inorganic paint containing a high organosilicon condensate, which lacks silanol groups, however. It is applied to an aluminum surface anodized in the usual way.
EP 1 780 313 A2 relates to an article, comprising a substrate having a surface of aluminum or an aluminum alloy, a sealing anodic coating layer overlying at least part of the substrate, and a layer of a silicon-containing polymer overlying the anodic sealing layer. According to the description, the coating is performed directly with the polymer, or with an aqueous solution of a silane without performing a cold or hot sealing directly following the preparation of the conversion layer. In this way, this is also shown in Example 1. However, reference is made to the military specification of the U.S. Department of Defense (MIL-A-8625F), according to which a complete sealing for at least 15 minutes (p. 7, items 3.8.1 and 3.8.1.1) is prescribed independently of layer thickness, however. The applied polymer coating is to be dried at a temperature of from 10 to 100° C.
In the motor vehicle field, there are a number of trim parts having surfaces of aluminum or aluminum alloys. Thus, WO 2009/068168 describes that the decorative surfaces are obtained by polishing or electropolishing. the most frequently used aluminum materials that are employed in the motor vehicle field are also known from this document. In addition to pure aluminum, these include aluminum alloys with the material symbols Al 99.85MgSi or AlMg0.5 or 0.8. The automobile manufacturers expect an alkali resistance of at least 11.5, and even up to 13.5 for particular components.
Appropriate alkali resistances and other properties of aluminum surfaces are prescribed, among others, by the manufacturer Volkswagen AG in their internal, but publicly available, Component Specification TL182 (issue 2011 January), “Inorganic Protective Layer on Aluminum Parts”.